Intravascular balloon expandable stents may reduce restenosis of saphenous vein grafts after PTCA. To determine the acute and chronic in vivo effects of a slotted tubular steel stent on view grafts we deployed 14 stents on balloon catheters in 14 femoral interposition vein grafts in 11 sheep. After allowing the vein grafts to arterialize over approximately 8 months, all stents were deployed and host responses were studied by angiography, light and scanning electron microscopy. At 3 weeks, 8 weeks and 5 months, we found no gross or histologic effects of intimal hyperplasia, vein graft erosion, late thrombosis, or progression of aneurysmal changes. In vein grafts with minimal residual stenosis and in animals given antiplatelet therapy, the patency was excellent (100%) and no deleterious host responses were noted. Thus, treatment of vein grafts with implantable stents is feasible, safe, and may improve long-term PTCA results.